This invention relates to a piston pump assembly used in hydraulic devices such as hydraulic pressure control devices, hydraulic pressure boosters and high-pressure pumps for pressure accumulators, and particularly to a piston pump assembly having a self-suction function which can by itself pressurize fluid to be drawn into a pump chamber and feed it into the pump chamber under pressure.
There is known a piston pump assembly of the type in which fluid (liquid) used as a pressure medium is pre-pressurized by a second pump, and re-pressurized by a first pump, and then discharged. A piston pump assembly of this type is disclosed e.g. in JP Patent Publication 2005-113880A (hereinafter referred to as “Patent document 1”).
The first pump of the piston pump assembly disclosed in Patent document 1 includes a first cylinder member and a first piston having one end thereof axially slidably inserted in the first cylinder member, while its second pump includes a second cylinder member, and a second piston axially slidably inserted in the second cylinder member.
The second piston is an annular piston axially slidably fitted in an annular groove formed in the outer periphery of the first piston,
With the piston pump assembly disclosed in Patent document 1, the second cylinder member is provided separately from the pump housing for the following reasons:    (1) Because the piston of the second pump, which is ordinarily called a “feed ring” (though it is referred to as a “slide ring” in Patent document 1), is made of rubber or a soft resin, by forming the second cylinder member from a resin, which has lower aggressiveness against the piston than the pump housing, which is made of a metal, it is possible to reduce wear of the piston.    (2) By using the second cylinder member to prevent an annular seal member which seals the outer periphery of the first piston, thereby liquid-tightly isolating a feed chamber of the second pump from a cam chamber kept at the atmospheric pressure, from being dropped out, it is possible to mount the annular seal member in a predetermined position without forming by recessing a seal member in the inner surface of the piston hole in which the first piston is inserted.
In conventional arrangements, the first and second cylinder members are fitted together with the first and second pistons mounted therein, and the thus assembled first and second cylinder members are mounted in the pump housing.
With this type of piston pump assembly, an annular seal surface of the second cylinder member is brought into close contact with an axially facing shoulder surface formed on the inner periphery of the piston hole, thereby isolating the feed chamber of the second pump defined between the outer periphery of the first piston and the inner periphery of the second piston from a fluid intake passage of the second pump formed in the outer periphery of the second piston. (This seal arrangement is called the “rod seal type”.)
With this arrangement, since the seal portion for isolating the feed chamber of the second pump from the fluid intake passage is defined by the shoulder surface on the inner periphery of the piston hole and the annular seal surface of the second cylinder member, in order to ensure required sealability, the shoulder surface and the annular seal surface have to be parallel to each other with high accuracy.
In this conventional arrangement, the second cylinder member is restricted by the first cylinder member. Also, since the second cylinder member is formed by molding, it is difficult to form the second cylinder member with high accuracy compared to when it is formed by cutting. For these reasons, the position of the second cylinder member tends to change, which may cause poor contact between the shoulder surface on the inner periphery of the piston hole and the annular seal surface of the second cylinder member. This could in turn lead to leakage of fluid in the feed chamber into the intake passage through the gap between these surfaces, thus lowering the volume efficiency of the second pump, and thus detrimentally influencing the discharge capacity of the entire pump assembly,
Also in the conventional arrangement, a step of fitting together the first and second cylinder members is necessary before mounting the second cylinder member in the pump housing, which reduces productivity and increases the manufacturing cost.